Anti-cholinergic agents prevent the passage of, or effects resulting from the passage of, impulses through the parasympathetic nerves. This is a consequence of the ability of such compounds to inhibit the action of acetylcholine (Ach) by blocking its binding to the muscarinic cholinergic receptors.
There are five subtypes of muscarinic acetylcholine receptors (mAChRs), termed M1-M5, and each is the product of a distinct gene and each displays unique pharmacological properties, mAChRs are widely distributed in vertebrate organs, and these receptors can mediate both inhibitory and excitatory actions. For example, in smooth muscle found in the airways, bladder and gastrointestinal tract, M3 mAChRs mediate contractile responses (reviewed by Caulfield, 1993, Pharmac. Ther., 58, 319-379).
In the lungs, muscarinic receptors M1, M2 and M3 have been demonstrated to be important and are localized to the trachea, the bronchi, submucosal glands and parasympathetic ganglia (reviewed in Fryer and Jacoby, 1998, Am J Resp Crit Care Med., 158 (5 part 3) S 154-160). M3 receptors on airway smooth muscle mediate contraction and therefore bronchoconstriction. Stimulation of M3 receptors localised to submucosal glands results in mucus secretion.
Increased signalling through muscarinic acetylcholine receptors has been noted in a variety of different pathophysiological states including asthma and COPD. In COPD, vagal tone may either be increased (Gross et al. 1989, Chest; 96:984-987) and/or may provoke a higher degree of obstruction for geometric reasons if applied on top of oedematous or mucus-laden airway walls (Gross et al. 1984, Am Rev Respir Dis; 129:856-870). In addition, inflammatory conditions can lead to a loss of inhibitory M2 receptor activity which results in increased levels of acetylcholine release following vagal nerve stimulation (Fryer et al, 1999, Life Sci., 64, (6-7) 449-455). The resultant increased activation of M3 receptors leads to enhanced airway obstruction. Thus the identification of potent muscarinic receptor antagonists would be useful for the therapeutic treatment of those disease states where enhanced M3 receptor activity is implicated. Indeed, contemporary treatment strategies currently support regular use of M3 antagonist bronchodilators as first-line therapy for COPD patients (Pauwels et al. 2001, Am Rev Respir Crit. Care Med; 163:1256-1276)
Incontinence due to bladder hypercontractility has also been demonstrated to be mediated through increased stimulation of M3 mAChRs. Thus M3 mAChR antagonists may be useful as therapeutics in these mAChR-mediated diseases.
Despite the large body of evidence supporting the use of anti-muscarinic receptor therapy for treatment of airway disease states, relatively few anti-muscarinic compounds are in use in the clinic for pulmonary indications. Thus, there remains a need for novel compounds that are capable of causing blockade at M3 muscarinic receptors, especially those compounds with a long duration of action, enabling a once-daily dosing regimen. Since muscarinic receptors are widely distributed throughout the body, the ability to deliver anticholinergic drugs directly to the respiratory tract is advantageous as it allows lower doses of the drug to be administered. The design and use of topically active drugs with a long duration of action and that are retained on the receptor or in the lung would allow reduction of unwanted side effects that could be seen with systemic administration of the same drugs.
Tiotropium (Spiriva™) is a long-acting muscarinic antagonist currently marketed for the treatment of chronic obstructive pulmonary disease, administered by the inhaled route.

Additionally ipratropium is a muscarinic antagonist marketed for the treatment of COPD.

Other muscarinic receptor modulators have been referred to. For example: U.S. Pat. No. 4,353,922 describes muscarinic modulators based upon a [2.2.1]azabicycloheptane ring system. EP418716 and US005610163 describe various [3.2.1]azabicyclooctane ring systems. WO06/017768 describes [3.3.1]azabicyclononane ring systems. [2.2.2]azabicyclooctane systems (quinuclidines) have been previously described, for example in US2005/0209272 and WO06/048225. [3.1.0]azabicyclohexane systems have been described in, for example in WO06/035282. [3.2.1]azabicyclooctane systems have been described in for example WO06/035303.
The class of β2 adrenergic receptor agonists is well known. Many known β2-agonists, in particular, long-acting β2-agonists such as salmeterol and formoterol, have a role in the treatment of asthma and COPD. These compounds are also generally administered by inhalation. Compounds currently under evaluation as once-daily β2 agonists are described in Expert Opin. Investig. Drugs 14 (7), 775-783 (2005). A well known β2-agonist pharmacophore is the moiety:

Also known in the art are pharmaceutical compositions that contain both a muscarinic antagonist and a β2-agonist for use in the treatment of respiratory disorders. For example, US2005/0025718 describes a β2-agonist in combination with tiotropium, oxotropium, ipratropium and other muscarinic antagonists; WO02/060532 describes the combination of ipratropium with β2-agonists and WO02/060533 describes the combination of oxotropium with β2-agonists. Other M3 antagonist/β2-agonist combinations are described in WO04/105759 and WO03/087097.
Also known in the art are compounds possessing both muscarinic receptor antagonist and β2-agonist activity present in the same molecule. Such bifunctional molecules provide bronchodilation through two separate modes of action whilst possessing single molecule pharmacokinetics. Such a molecule should be easier to formulate for therapeutic use as compared to two separate compounds and could be more easily co-formulated with a third active ingredient, for example a steroid. Such molecules are described in for example, WO04/074246, WO04/089892, WO05/111004, WO06/023457 and WO06/023460, all of which use different linker radicals for covalently linking the M3 antagonist to the β2-agonist, indicating that the structure of the linker radical is not critical to preserve both activities. This is not surprising since the molecule is not required to interact with the M3 and β2 receptors simultaneously.